1. Field of the Invention
The present invention relates to organic compound crystals and field-effect transistors.
2. Description of the Related Art
During recent years, research has been actively carried out on organic transistors, and the performance of the organic transistors has almost reached a practical level. Currently, it has been reported that 2,3,6,7-dibenzanthracene (also referred to as “pentacene”), which is a condensed aromatic compound, shows the best performance as the material for a channel-forming region in an organic transistor (for example, refer to H. Klauk et al., J. Appl. Phys. 92, 5259 (2002)).
With respect to the electron state of pentacene, band analysis is performed using ab initio calculation (for example, refer to M. L. Tiago et al., Phys. Rev. B67, 115212 (2003)). The pentacene molecules in a crystal do not have a stack structure as shown in FIG. 22, but have a herringbone structure as shown in FIG. 23. Two-dimensional layers each having the herringbone structure are stacked (for example, refer to R. B. Champbell et al., Acta Cryst. 14, 705 (1961); and D. Homes et al., J. Eur. Chem. 5, 3399 (1999)). Band analysis results support that a two-dimensional conduction path is formed in the two-dimensional layer.
In a one-dimensional conduction band which is often observed when molecules have a stack structure, the conduction path is anisotropic (one-dimensional). Therefore, such molecules are disadvantageous in the formation of a channel-forming region in a field-effect transistor. Moreover, because of the one-dimensional conduction band, interactions among charge carriers (e.g., Coulomb repulsive force among the charge carriers) are large, and thus movement of charge carriers is inhibited.
Consequently, the two-dimensional conduction band in pentacene is desirable as the electron structure for constituting a channel-forming region of a field-effect transistor. Since pentacene is a p-type substance, holes are accumulated in the HOMO band, contributing to conduction.
One of the parameters which represent the movement of charge carriers is mobility. The mobility is defined as the drift velocity of charge carriers per unit electric field. Higher mobility enables higher speed movement of charge carriers, and as a result, high speed performance of the field-effect transistor is enabled. However, it is difficult to directly evaluate the mobility by calculation. A parameter alternative to the mobility is the effective mass of the band. The degree of ease of movement of charge carriers in the band is expressed based on the effective mass of the band. Charged carriers present in the band having the smaller effective mass have higher mobility. In order to decrease the effective mass, the bandwidth must be large, and furthermore, interactions among molecules must be large. For the reasons described above, in an attempt to form a higher performance channel-forming region using an organic material, it is necessary to design molecules capable of exhibiting strong interactions among each other two-dimensionally or three-dimensionally in the crystalline state.
Interactions among molecules are carried out by π-electron systems extending perpendicular to the σ-bonds constituting the backbones of molecules. Furthermore, to enable free movement of charge carriers in a molecule, the π-electron systems must be conjugated and extend intermolecularly.